1. Field of the Invention
The present invention relates to a tunnel magnetoresistance device.
2. Description of Related Art
There are a variety of electronic devices that utilize the magnetoresistance effect. MRAMs (Magnetoresistive Random Access Memories), magnetic heads, and magnetic sensors head the list of such electronic devices.
Electronic devices utilizing the GMR (Giant Magnetic Resistance) effect arising from the magnetic exchange coupling between laminated magnetic thin films are also being put into commercial mass production.
Meanwhile, the TMR (Tunnel MagnetoResistance) device is expected to provide highly sensitive and highly accurate electronic devices, because the MR ratio (Magnetic Resistance change ratio) of the TMR device is far larger than that of the GMR device.
As disclosed in Japanese Patent No. 3331397, for example, the TMR device has a structure including a lower electrode layer formed on a substrate, a pinned layer (fixed magnetization layer) formed on the lower electrode layer by laminating one or more magnetic material layers, a tunnel barrier layer made of nonmagnetic insulating material formed on the pinned layer, a free layer (free magnetization layer) formed on the tunnel barrier layer, and a metal electrode layer formed on the free layer.
The direction of the electron magnetic spin in the pinned layer is fixed, while that in the free layer easily inverts depending on an external magnetic field.
With such a TMR device, since a direct tunneling current flowing through the tunnel barrier layer largely changes depending on the energy potential between the pinned layer and the free layer, it is possible to measure the external magnetic field with high precision.
The direct tunneling current is affected by the properties of the material and the thickness of the tunnel barrier layer. That is because the tunnel barrier height, which determines the intensity of the electric field within the tunnel barrier layer, depends on the properties of the material and the thickness of the tunnel barrier layer. This is explained in more detail below. The tunnel junction resistance Rσ of the TMR device is given by the following expressions (1) and (2).Rσ=Cσ−exp(2κd)  (1)κ=(2mφ/h2)1/2  (2)
In the above expressions, Cσ is a value depending on electron states of the tunnel barrier layer and the magnetic layers putting the tunnel barrier layer therebetween, d is a barrier thickness of the tunnel barrier layer, φ is a barrier potential height from the Fermi potential, m is the mass of electron, and h is the Planck's constant.
As is evident from the expressions (1) and (2), the tunnel junction resistance Rσ decreases exponentially with reduction of the barrier thickness (or the thickness of the tunnel barrier layer).
It is desirable to make the thickness of the tunnel barrier layer as small as possible, because the MR ratio and the tunneling current increase with reduction of the tunnel junction resistance Rσ. In the TMR device, the MR ratio is defined as a tunnel junction resistance ratio between when the electron spin direction in the free layer is the same as that in the pinned layer and when the electron spin direction in the free layer is opposite to that of the pinned layer. Generally, the thickness of the tunnel barrier layer has to be smaller than 5 nm, desirably, 1 nm.
Generally, the tunnel barrier layer is a thin metal oxide film formed by a process including the steps of forming a metal film as thin as about 1 nm by PVD (Physical Vapor Deposition) method, for example, by sputter deposition, and oxidizing the formed thin metal film through heating processing, plasma processing, or air exposure in an oxidation atmosphere.
However, there is a problem in that the conventional tunnel barrier layer formed by PVD method lacks uniformity in layer thickness and layer quality.
It arises from the fact that an island-growth phenomenon easily occurs during a process of forming a thin metal film having a thickness of the order of about 1 nm, which is equivalent to the total height of several to several tens of a molecular layer, and as a result, metal clusters are scattered over the surface of the formed thin metal film.
For such reason, the thickness of the tunnel barrier layer formed by PVD method is not uniform even if the formed tunnel barrier layer is subjected to oxidizing processing. This may cause local electric field concentration, a leak current, tunneling current variation, and pinholes within the tunnel barrier layer. These significantly degrade the performance of the TMR device.
Incidentally, it is known that a lamination of a ferromagnetic film and an antiferromagnetic film has a high coercivity because of the effect of exchange coupling therebetween. Accordingly, the pinned layer, which is required to have a high coercivity, is preferably a lamination of thin metal films. However, in this case, it is very difficult to control surface roughness of the pinned layer, because the pinned layer is formed by a process including the step of forming a thin metal film. If the surface roughness of the pinned layer is large, the drapability of the tunnel barrier layer over the pinned layer is lowered.
Several measures have been proposed for providing the pinned layer having a smooth surface. Such measures include forming the pinned layer by epitaxial growth or heteroepitaxial growth so that it has nearly a monocrystal structure, and smoothing the surface of the formed pinned layer by electron beam polishing. However, it is difficult to carry out such measures from economical and technical standpoints.
Furthermore, a surface phase tends to be formed when forming a thin metal film as the tunnel barrier layer on the pinned layer due to intermetallic diffusion therebetween. Such a surface phase makes it difficult to control the layer thickness of the tunnel barrier layer.
Incidentally, the TMR device is expected to be small in size for miniaturization of electronic circuits. However, as the size of the TMR device becomes smaller, the tunneling current and sensitivity thereof becomes smaller as can be seen from the following expression (3)Itun=J·S  (3)
where Itun denotes a tunneling current, J denotes a density of the tunneling current, and S denotes an effective surface area of the TMR device.
Since the tunneling current is proportional to the effective surface area of the TMR device, it is possible to increase the tunneling current without increasing the surface area of the TMR device by venturing to make large the surface asperities of the pinned layer, thereby making large the effective surface area.
However, if the surface asperities of the pinned layer are made larger, the drapability of the tunnel barrier layer over the pinned layer becomes worse, and accordingly the performance of the TMR device is worsened.